92168.pdf

44 Kinetic properties

ratio a^2 /(a+2b) small and allow the Donnan effect to be virtually

eliminated.

Rotary Brownian motion

In addition to translationai Brownian motion, suspended molecules

or particles undergo random rotational motion about their axes, so

that, in the absence of aligning forces, they are in a state of random

orientation. Rotary diffusion coefficients can be defined (ellipsoids of

revolution have two such coefficients representing rotation about

each principal axis) which depend on the size and shape of the

molecules or particles in question^28.

Under the influence of an orientating force partial alignment of

asymmetric particles takes place, which represents a balance between

the aligning force on the particles and their rotary diffusion. The

system will therefore become anisotropic. It is possible to draw

conclusions concerning particle dimensions by studying various

changes in physical properties brought about by such particle

alignment.

Streaming birefringence^35

The sample is subjected to a strong velocity gradient - for example, in

a concentric cylinder viscometer (Figure 2.12) - and the resulting

molecular or particle alignment causes the previously isotropic

solution to become doubly refracting (birefringent). The magnitude

of the birefringence is related in an elaborate theory to the rotational

diffusion coefficient, and, hence, to the molecular or particle

dimensions.

In certain cases (e.g. iron(III) hydroxide sol) birefringence can be

produced by the aligning action of electrical or magnetic fields.

Dielectric dispersion

When a solution containing dipolar molecules is placed between
electrodes and subjected to an alternating current, the molecules
tend to rotate in phase with the current, thus increasing the dielectric
constant of the solution. As the frequency is increased, it becomes
more difficult for the dipolar molecules to overcome the viscous